1. Field of the Invention
This invention relates to processes for hydraulically expanding a conduit surrounded by a structure in order to bring the conduit into contact with, or engagement with, the surrounding structure. It finds particular application in reducing the clearance between heat exchange tubes and baffle plates in nuclear steam generators, and in joining reinforcing sleeves on the inside walls of these tubes by producing interference joints therebetween.
2. Description of the Prior Art
Processes for hydraulically expanding plastically-deformable conduits are known in the prior art. Such hydraulic expansion processes are frequently used to effect repairs or maintenance on the heat exchanger tubes of a nuclear steam generator. In such generators, it is generally difficult to gain access to the outside tube surfaces due to the density in which they are arranged, and the limited access space afforded by the few water inlets and outlets in the walls of these generators. Therefore, the most convenient way to gain access to these tubes is through their inlet ports which are present in the tubesheet dividing the primary side of the steam generator from the secondary side. Accordingly, when the walls of these tubes have been weakened or pitted by corrosion or excessive heat and fluid currents, sleeving procedures have been developed wherein a stainless steel reinforcing sleeve is concentrically inserted inside the tube, slid to the axial portion of the tube which has been weakened or pitted, and joined to the inside of the tube by expanding the ends of the sleeve into the walls of the tube in order to form an interference-type joint between the sleeve and the tube. Typically, the hydraulically formed joint is then internally cold-rolled with a conventional cold-rolling tool in order to strengthen the joint, and to sealingly engage the outside walls of the sleeve at the joint against the inside walls of the tube. The end result of this known process is that the corroded or pitted portion of the heat exchange tube is mechanically reinforced with an internal water shunt which effectively diverts the flow of water away from the weakened walls of the tube and through the walls of the sleeve.
Unfortunately, the application of prior art tube expansion processes to the maintenance of the heat exchanger tubes of a nuclear steam generator is not without material shortcomings. For example, no provision is made in prior art tube expansion processes to consider the specific elastic and plastic properties of the tubes being expanded. Instead, these processes attempt to create interference fittings or other expansions on the basis of preselected "average" elastic and plastic properties of the tubes being expanded. Hence, it is difficult to obtain truly uniform expansions for interference joints, or any other tube expansion performed incident to a maintenance procedure. Since mechanical reliability is of paramount importance in a nuclear steam generator, such non-uniformity and the uncertainty of results which attends it is undesirable.
Clearly, a need exists for a tube expansion process which is capable of producing highly uniform expansions in order to maximize the mechanical reliability of the system as a whole. Ideally, such a process should consider the specific elastic and plastic properties of the tube being expanded so that a nearly perfect expansion is possible in each tube.